75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

237 Chilling Stress is Associated with ROS Overload in Roots and Leaves
John Einset
Norwegian University of Life Sciences
Although normally considered as chilling-tolerant, Arabidopsis plants exposed to 2 days of chilling at 4 C show
elevated levels of reactive oxygen species (ROS) and inhibited root growth for up to 4 days when transferred back to
optimal growing conditions. During this recovery period, ROS levels decline in root tips and in leaves. If plants are
pretreated with glycine betaine (GB) prior to the chilling treatment, ROS levels do not increase during chilling and
optimal growth begins as soon as plants are transferred back to normal growing conditions without a recovery period.
Using microarray technologies, we can show that GB up-regulates several genes in both roots and leaves that reinforce
intracellular processes protecting cells from oxidative damage and others that appear to be involved in setting up a
scavenging system for reactive oxygen species in cell walls. In roots, GB-activates genes for transcription factors,
membrane trafficking proteins (RabA4c, RabB1b), cell wall peroxidases (ATP3a, ATP15a), superoxide dismutases in
the cytoplasm, plastids and mitochondria, a mitochondrial catalase, the root specific NADPH-dependent ferric reductase
(FRO2) localized to the plasma membrane as well as glutathione and ascorbate metabolizing enzymes in the cytoplasm
and cell wall. Genes activated in leaves include transcription factors, several intracellular ROS metabolism enzymes as
well as membrane trafficking components. In addition, specific extracellular peroxidases are activated by GB in leaves
as well as a plasmamembrane NADPH-dependent ferric reductase (FRO6). Experiments with knockout mutants provide
direct evidence that two of the GB-activated genes, one gene coding for a membrane trafficking protein (RabA4c) and
the other coding for a putative bZIP transcription factor, are required for GB’s effects on recovery from chilling and ROS
accumulation during chilling. GB does not prevent chilling stress in these knockouts. Experiments with RabA4c promoter-
GUS and -YFP transgenics show that RabA4c expression coincides with regions of most active ROS accumulation in
vascular tissues during chilling stress. Taken together, these results plus the fact that application of ROS directly to plants
in the absence of chilling can cause root growth inhibitions suggest that ROS may be the cause of chilling stress.
238 An Arabidopsis thaliana MAPK (At3g45640/AtMAPK3) is involved in glucose and ABA
responses
Jaime Aportela Cortez, Patricia Leon Mejia, Angel Arturo Guevara-Garcia
Instituto de Biotecnologia, UNAM Cuernavaca, Mor., Mexico
Mitogen activated protein kinases (MAPKs) signaling cascades are involved in the control of abiotic and biotic stress
responses in all organisms. Glucose plays an important role as carbon source for plant growth and development, but also
as signal to regulate metabolism, differentiation and stress responses. Abscisic acid (ABA) participates in the control of
seed germination and mediates plant responses to some kind of abiotic stress. The co-regulation of several plant genes
by both glucose and ABA has been reported, but although the cross-talk between this two signal transduction pathways
is experimentally supported, little is known about the molecular basis of their interaction. Previously has been reported
that compared with wild-type plants, Arabidopsis thaliana transgenic lines overexpressed a stress-activated MAPK
(At3g45640/AtMPK3) were more sensitive to ABA-triggered postgermination growth arrest (1). Here, we compare the
performance of A. thaliana transgenic lines with high (35ScaMV-AtMPK3), and low (RNAi/AtMPK3) levels of expression
of the AtMPK3 gene. Our results show than ABA-hypersensitive AtMPK3 overexpressed lines are also glucose-tolerant,
whereas AtMPK3 suppressed lines are lightly ABA-tolerant and apparently glucose-sensitive. Those findings implicate
that both ABA and glucose signaling pathways, seems to converge in a stress-activated MAPK cascade, in which AtMPK3
is involved. Our studio includes: a) the phenotypic characterization in different developmental stages of transgenic lines
growing in ABA and glucose; b) an analysis of expression of the AtMAPK3 gene in response to ABA and glucose on
wild-type plants; and c) an analysis of expression of some ABA/glucose responsive genes, on wild-type and transgenic
plants, treated with ABA and glucose.
1) Lu, C., Han, M-H., Guevara-Garcia, A. and Fedoroff, N. (2002). Mitogen-activated protein kinase signaling in post-germination arrest of
development by abscisic acid. PNAS, USA. , 99 (24):15812-15817.